The present invention relates to a device for limiting the speed of an exhaust-gas turbocharger of an internal combustion engine, a pressure sensor measuring the boost pressure (supercharging pressure) on the pressure side of a compressor of the turbocharger arranged in the induction pipe of the engine, and provision being made for means which reduce a preselected setpoint boost pressure when the speed (rotational speed) reaches a maximum permissible value.
When designing an exhaust-gas turbocharger and its control, care must be taken that the supercharger speed does not exceed a maximum permissible value. As can be gathered from xe2x80x9cMTZ Motortechnische Zeitschriftxe2x80x9d [Motor Engineering Journal] 53 (1992), 10, pages 454-462, the boost pressure is controlled in a known manner by comparing a setpoint boost pressure resulting from the driver""s command to an actual boost pressure measured by a boost pressure sensor in the induction pipe. For monitoring the supercharger speed, usually a speed sensor is used which detects the speed of the compressor in the induction pipe. By comparing the measured supercharger speed to a threshold value which corresponds to maximum permissible speed value, it can determine whether the turbocharger reaches a critical speed range and act upon the boost pressure control correspondingly. From the mentioned MTZ it follows that the boost pressure in the induction pipe is always adjusted to programmed values which were determined in standard atmosphere. With increasing altitude and, consequently, dropping pressure, therefore with decreasing air density, the demanded setpoint boost pressure is only attained by increasing the supercharger speed. To this end, the wastegate of the turbocharger is closed to a greater degree and, consequently, the turbine is charged with a larger quantity of exhaust gas. Using an atmospheric pressure sensor, the setpoint boost pressure preselected by the boost pressure control can be lowered in the case of dropping ambient pressure, thereby also allowing the supercharger speed to be limited below its maximum permissible value.
An object of the present invention is to provide a device for limiting the speed of an exhaust-gas turbocharger, the device neither requiring a speed sensor for the supercharger speed nor an atmospheric pressure sensor.
The mentioned objective is achieved by providing a bandpass filter which filters out a spectral component from the output signal of a boost pressure sensor existing in the induction pipe, the spectral component appearing in the signal spectrum of the boost pressure sensor when the air column in the induction pipe is set into a vibration which develops when the compressor of the turbocharger rotates at the maximum permissible speed. Here, a criterion for limiting the supercharger speed below its maximum permissible value is derived from the output signal of a boost pressure sensor which exists anyway. In this context, a speed sensor and an atmospheric pressure sensor can be dispensed with.
Accordingly, a correcting element exists which compares the spectral component delivered by the bandpass filter to a threshold value and, in the case that the threshold value is exceeded, generates a correction signal which reduces the preselected setpoint boost pressure.
The boost pressure sensor is expediently a piezoelectric sensor since a piezoelectric element responds to vibrations of the air column in the induction pipe in a very sensitive manner.